This invention relates to an improvement of a thickness gauge wherein the thickness of sheets of various materials is measured by measuring the amount or intensity of the radiations transmitting through the sheets.
Thickness gauges of this type have been used extensively in various applications because it is possible to continuously measure the thickness of web shaped material without contacting the same. Especially, where a source of X-rays is utilized as the radiation source, since the noise caused by the radiations is small and since the response is fast, X-ray radiation gauges are widely used for on line measurement in rolling mill lines of metals, such as steel and aluminum and for the automatic adjustment of the gauge.
However, in the prior art X-ray thickness gauge the attenuation characteristic of the substance varies in accordance with the energy of the X-ray generator or the radiation source, so that the variation with time in the output of the X-ray generator, that is the so-called drift causes errors in the measured value of the sheet gauge. Moreover, the sensitivity of the detector and the constants of the associated electrical circuits also vary due to aging and variation in the external conditions and these variations also cause errors in the measured value.
According to a prior art thickness gauge shown in FIG. 1, for the purpose of improving the measuring accuracy, a reference sheet 11 is provided which is used to calibrate the thickness gauge each time the measurement is made. More particularly, a preset signal is applied to a gauge setting device 12 and a gauge correction device 13 so that an operator or an electronic computer 14 performs an operation in accordance with an equation which determines a reference sheet having an amount of absorption of the radiations which is equivalent to the thickness of a sheet or sample to be measured, thus applying a control signal to driving means 15 for inserting the reference sheet corresponding to the input signal across the X-ray flux radiated from a X-ray generator 21. The X-rays attenuated by the reference sheet are received by a radiation detector 16 for producing an electric signal which is converted into a signal proportional to the thickness of the reference sheet by the action of a non-linear amplifier 17. The output from this amplifier is compared with the output of the gauge setting device 12 in a comparator 18 and the output thereof is applied to an indicating meter 19 through an amplifier 20. Further, a servo-system 10 is provided for driving a member (not shown) for varying the energy of the X-rays generated by the X-ray generator 21 or a member (not shown) for adjusting the intensity of the X-rays as shown by dotted lines, thus calibrating the X-ray gauge meter. Thereafter, a sheet to be measured 22 is inserted across the X-ray flux instead of the reference sheet 11 for measuring the thickness of the sheet 22.
However, in the radiation thickness gauge just described since the operation parameters of the thickness gauge are adjusted to a calibrated state by the servo-system for providing the zero point calibration as well as the sensitivity calibration the adjustment of the operation parameters by the servo-system causes errors in the measured thickness.